With the use of composite materials becoming more prevalent within the aircraft industry, it has become apparent that issues relating to the control of interfaces between components of large, complex, composite structures have become more of a challenge. In the case of an aircraft wing, the interfaces between wing spars, ribs and covers (skins) for a commercial airliner, for example, are particularly difficult to control. This is primarily due to the difficulty in controlling the through thickness tolerance associated with complex laminate composite components, especially when there are ramps (thickness variations) within these components.
This problem is a function of both the through thickness tolerance of individual plies within the laminate, as well as the location of the ply edges, which varies both the location and angle of the ramp. This can cause significant issues with respect to the interface profile of the components, and can lead not only to problems during the manufacturing phase, requiring extensive interface machining to generate the desired interface profile, but also during the design phase of the components.
A traditional sandwich panel has a constant thickness core layer used to afford structural rigidity. Previously, there has been a tendency to import traditional aircraft design philosophies, as developed for metallic aircraft construction, into composite aircraft design. In the case of an aircraft wing cover, reinforcing elements such as spanwise stringers have traditionally been integrally formed with an outer surface of a composite sandwich panel.
The highly complex ramped surface created by integrally forming the stringers with the laminate skin forms the interfacing surface of the cover with the ribs and spars. Through thickness tolerance issues associated with a complex ramped surface makes it very difficult to control the profile of this critical interfacing surface during manufacture. Also, variations in the cover/stringer design during the wing design phase directly impact on the dimensions of the ribs and spars. Therefore, the design of the spars and ribs cannot be finalised until the design of the cover/stringers, and thus the interface, has been finalised. This has a significant impact on the potential for concurrent design of the wing structure as a whole. This leads to increases in the overall design cycle of the components and hence additional cost.